fix: move PR template to default location (#136)

`.github/PULL_REQUEST_TEMPLATE/*.md` [is supported to host _multiple_
pull request
templates](https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository#adding-a-pull-request-template),
but they must be explicitly selected [via a URL Query Parameter
](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/using-query-parameters-to-create-a-pull-request).
This moves the template in to the default location so that it will be
applied to all PRs.

Signed-off-by: GitHub <noreply@github.com>